As the use of multiple-electrode systems has increased, so has the need to continuously sense the condition of leads which connect a patient to a piece of equipment, such as a medical diagnostic or therapeutic apparatus. A lead, as discussed herein, comprises an electrode and a conductor connecting the electrode to the piece of equipment. The loss of electrical continuity in the lead between a patient and a diagnostic or therapeutic apparatus usually occurs as an electrode becomes separated from a patient's skin, and is referred to as a leads-off condition. Such a condition in a multiple-electrode system will cause the diagnostic or therapeutic apparatus to receive erroneous and/or incomplete electrical signals from the patient. Thus, it is important to detect a leads-off condition quickly so that someone, such as a medical technician, can correct the situation.
An example of an apparatus in which quick detection of a leads-off condition is paramount is a cardiac monitor that senses the ECG signal of a patient in order to determine the patient's cardiac activity and that senses the patient's transthoracic impedance in order to determine the patient's pulmonary activity. As is well known, a patient's cardiac activity and pulmonary activity must both be determined in order to decide upon an appropriate therapeutic measure, such as defibrillation. For example, if the normal sinus rhythm in a patient's ECG signal is absent, and the transthoracic impedance signal indicates an absence of breathing, a logical step would be to defibrillate the patient so as to stimulate the patient's heart. Contrariwise, if the normal sinus rhythm in the ECG signal is absent but the transthoracic impedance signal indicates the patient is still breathing, then the patient should not be defibrillated.
A method commonly employed by the prior art for detecting a leads-off condition is to sense lead impedance. The lead impedance actually comprises several impedances, such as an electrode impedance and a conductor impedance. Another impedance that makes up a significant part of the lead impedance is formed at an electrode-to-patient's skin connection. A leads-off condition that is produced by an electrode being detached from a patient produces a high electrode-to-skin connection impedance and therefore a high lead impedance.
One approach used in the prior art to sense lead impedance is to apply a small DC signal to the leads and to compare the resulting DC voltage to some threshold level. Typically, a DC voltage that equals or exceeds the threshold level is indicative of a leads-off condition. One such method is found in U.S. Pat. No. 4,577,639 (Simon et al.). A problem associated with this type of leads-off detection system is that the DC voltage that is used to sense the lead impedance may corrupt a physiological electrical signal (such as an ECG signal) present on that particular lead.
Another approach used by the prior art to detect a leads-off condition is to supply a high-frequency constant current signal to the leads. A return AC signal is demodulated and filtered to remove the physiological electrical signal (such as an ECG signal). The resulting signal is then compared to a threshold level to determine whether or not a leads-off condition exists. The demodulated signal may be further amplified and filtered to remove any DC components, thereby producing a signal that may be used to measure a patient's transthoracic impedance. Such an approach is found in U.S. Pat. No. 4,610,254 and its divisional U.S. Pat. No. 4,619,265 (Morgan et al.). One problem associated with this approach is that it is limited to systems using a single pair of electrodes.
Another problem associated with the prior art method of measuring a patient's transthoracic impedance is related to the size of the signal used and the size of the prior art apparatus. Typically, the prior art apparatus employ a transformer to produce a high frequency signal that is applied to the appropriate leads. Such a transformer is relatively large and inhibits miniaturization of the apparatus. This is an important consideration with portable apparatus, such as portable, multiple ECG lead apparatus. Also, the amplitudes of the high frequency signals produced by the transformers in the prior art are typically too large for many electronic components used with the compact portable apparatus.
As can be readily appreciated from the foregoing discussion, there is a need for a method and apparatus that will provide both lead impedance and transthoracic impedance information for multiple-electrode systems (i.e., systems with two or more electrodes) without corrupting the physiological electrical signals present. Additionally, the apparatus should be small enough to be used with portable, multiple-electrode systems. The present invention is directed to a method and apparatus that provides these results.